Anti-kickback devices are installed on two types of machines: square edging machines which are provided with two or more circular saws rotating against the advance of lumber and having a variable spacing therebetween, and ripsawing machines which are provided with multiple circular saws spaced from one another at fixed intervals. Most of the latter machines have two saw-carrying shafts, one extending above the lumber, the other below. Each saw on the shaft extending below the lumber extends in a vertical plane that includes a corresponding saw on the shaft extending above the lumber. The saws disposed beneath the lumber rotate against the advance of lumber while those above rotate with the advance of lumber.
Square edging machines must be equipped with anti-kickback devices since the saw-carrying shaft rotates against the advance of the lumber which can be projected outwardly towards the operator at the feeding end of the machine.
The ripsawing machines can also project lumber towards the operator at the feeding end due to the rotational direction of the shaft located beneath the lumber as well as towards the exit end of the machine due to the rotation of the upper saws. This latter type of machine, therefore, requires anti-kickback devices at its entrance.
However, because anti-kickback devices are designed to protect against the projection of particles or full lumber pieces towards the operator, they have no use in protecting against projections from the outlet of the machine and caused by the saws located in the upper part of the machine.
Anti-kickback devices have two functions: first, they provide a curtain wall that surrounds the lumber piece that enters the machine thereby performing a blocking or sealing function to confine, inside the machine, all the lumber fragments as well as saw teeth which may break and be projected exteriorly towards the operator. To achieve this sealing function, it is required that fingers that form part of anti-kickback devices are arranged to have their contour follow as closely as possible the contour of the passing lumber to thereby minimize openings to the outside. These anti-kickback fingers must be narrow and return easily on the lumber to their closed position; they must have very little spacing between them.
This sealing function cannot be adequately obtained with a single series of anti-kickback fingers due to the various configurations of lumber pieces passing through the machine, especially those thick lumber pieces having irregular shapes.
Hence, a second series of anti-kickback fingers situated under the lumber is installed on machines, especially used for ripsawing thick lumber.
Still, two series of anti-kickback fingers are often still not sufficient to provide an adequate sealing action on ripsawing machines due to the power of these machines, to the various configurations of lumber pieces and to operating and feeding conditions.
Most machines are therefore equipped with a third series of anti-kickback fingers which are situated at the feeding end of the machine, over the lumber. A lumber piece that will kickback from the rotating saws to the entrance of the machine, with contact and pass below the first series of fingers, raise and pass above the second series of fingers and then contact and pass below the third series of fingers.
The vertical distance that extends from the tip of one series of fingers to the tip of the successive series of fingers is called the "overlap". Sealing is better obtained if the overlap is substantial. This overlap is usually expressed in percentage of the maximum thickness of the lumber which passes through the machine (ex. 75% of 8 inches).
In addition to sealing, another feature required on an antikickback finger is its ability to bite into and retain the lumber pieces, which would otherwise kickback. In order to adequately achieve this function, the fingers must contact the lumber with an angle which varies relative to the shape and the wear of the tip as well as to the physical and mechanical properties of the lumber. Beyond a predetermined tip wear, the action of a finger depends solely on the coefficient of friction between the finger extremity and the lumber. The coefficient of friction therefore determines the minimum contact angle required. In order to accomplish this second function, the fingers must be strongly built in order to oppose the kickback forces. They must have a sharp extremity as well as maintain, for various thicknesses of lumber passing by, an angle which is equal or less than the angle corresponding to the coefficient of friction between the tip and the lumber.
It is recognized that presently used fingers provide inadequate sealing due to various reasons, such as a low tip overlap, a large spacing between the fingers at the level of the tips and the general configuration of the fingers.
Various unsuccessful attempts have been made to increase the reliability of fingers with respect to their function of "biting and retaining". Principal causes are the wrong angles of lumber attack with which they must work and the rapid wear of the tips. Exception being made with lumber pieces of small thicknesses (approximately 3 and 4 inches) for finger tips located above the lumber, none of the fingers operate with angles which are less than the corresponding angle of coefficient of friction between a worn tip and lumber. The second series of fingers located underneath the lumber operate with an angle of attack which is largely greater than the angle corresponding to the coefficient of friction between the lumber and the tip. Therefore, it is rare that these fingers effectively operate in conditions to "bite and retain" the lumber (which is then kicked back).
Conclusively single tip fingers cannot perform adequately both functions, sealing and "biting and retaining" simply because both functions are incompatible with each other; if a finger adequately performs the sealing function, it is then too long to perform the "biting and retaining" function and vice versa, if a finger performs adequately the "biting and retaining" function, it is too short to perform adequately the sealing function.
The extremities of the fingers presently used wear out rapidly. It s extremely difficult to sharpen them on location or to replace them since the entire finger assembly must be dismounted. With the exception of the extremities of the second series of fingers, all others wear according to various angles depending on the thickness of the lumber which passes in the machine; this thickness varies from one lumber to another.
The extremities of the second series of fingers wear out to a shape which is somewhat more rounded than those of the other two series as they operate constantly under the same angle of attack.